All modern infrared spectrophotometers comprise optical multiple pass long-path systems. However, we are now witnessing the advent of new spectroscopy based on the use of high-intensity lasers and superhigh resolution laser semiconductor spectrometers. In these conditions, conventional multiple pass optical systems utilizing the classical White absorption cells are definitely obsolete since they cannot provide a large enough number of passes in the system.
Multiple pass optical systems showing great promise at the present stage of development are based on image matrices on arrays. But the existing matric systems are extremely complicated and unreliable.
Known in the art is a multiple pass optical matrix system (cf., for example, Journal of the Optical Society of America, 66, No. 5, May 1976, John U. White, Very Long Optical Paths in Air, pp. 411-416) comprising three objective mirrors arranged at the input and output of the laser beam, two main field mirrors having the radius of curvature equal to that of the objective mirrors and positioned at a distance equal to the radius of curvature of the latter, and also two diagonal mirrors having the total radius of curvature equal to the radius of curvature of an individual field mirror, arranged at an angle close to a right angle and adjoining one of the main field mirrors. In this system, each field mirror is individually mounted and can be rotated.
But this system is deficient in that the objective mirrors are individually mounted and rotated and the errors accumulate with the number of passes. The system, therefore, is extremely unstable, which is particularly felt if the number of passes is large.
Besides, the system comprises diagonal field mirrors operating at large incident angles, which results in greater astigmatism and, consequently, larger sizes of images.
One more deficiency consists in that the diagonal field mirrors produce additional reflecting surfaces impairing the translusency of the system.
This system is extremely complicated in design and adjustment, which affects its operational characteristics.
And, finally, the system can only operate with a coherent radiation source whose beam divergence is very small, which is a serious limitation to its application field.
Also known in the art is a multiple pass optical matrix system (cf., for example, P. L. Hanst, Advances in Environmental Science and Technology, vol. II, ed. by J. N. Pitts and R. L. Metcalf, Wiley, NY, 1971, pp. 160-165) comprising at least four objective mirrors, at least four field mirrors whose radii of curvature are equal to those of the objective mirrors, and which are arranged at a distance equal to the radius of curvature from said objective mirrors on the side of the entrance and exit apertures, and a means for coupling light beams out of the cell. In this system objective mirrors have individual adjustments, and one mirror can be rotated.
But this system is deficient in that individually adjustable objective mirrors and one angularly displacable mirror to change the number of passes can be the cause of error accumulation. The system is not vibration proof. The recommendation to cast the mirror array in epoxy cement produces a single purpose system which cannot be adjusted for any other application.
Besides, in this system each field mirror produces only one row of intermediate images, which is a limitation to the measuring range. To expand the measuring range by increasing the number of passes, more objective and field mirrors should be added, the total number of mirrors must be a multiple of four. Thus, a system for 220 passes and 10 rows of images in the matrix should have a total of 20 objective and field mirrors. The system becomes extremely complicated to operate and, consequently, unreliable.
Also known in the art is a multiple pass optical matrix system (cf., for example, U.S. Pat. No. 3,726,598, Cl. G OIJ 3.02, Apr. 10, 1973) wherein the radiation flux from an illumination source enters through an inlet window of the cell housing to hit one of the two rigidly secured main objective mirrors having the same radii of curvature and mechanically connected to a mount which is secured to a means for rotating said mirrors about an axis perpendicular to the row of images on the matrix, and further hits a main field mirror whose radius of cruvature is equal to that of the main objective mirror and which is arranged along the longitudinal axis of the system at a distance equal to the radius of curvature of the main objective mirrors, wherefrom the radiation flux is directed to another main objective mirror, is reflected therefrom to the main field mirror, and is once more directed to the first main objective mirror, and in the last pass leaves the system through the exit window.
This system is deficient in that the image matrix on the main field mirror has only two rows, thus limiting the number of passes and, therefore, the length of the optical path which can be achieved in this system.